Figure 30.4 :Structure of 70s Ribosome
Activation of amino acid: During this process amino acids are attached to the t-RNA in the presence of enzyme Amino acyl-t-RNA synthetase, this enzyme activate the amino acids by attaching covalently to the t RNA, when t RNA get charged, its named as aminoacyl-t RNA. During this process amino acids are attached to t-RNA with high energy bond, so called as activated amino acids.
Initiation: In eubacteria, first amino acid in the polypeptide chain is N formyl-methionine which is specific to three codes as AUG, GUG, and UUG. Whenever these codes are present at the initiation point they code for N-formyl-methionine but if they are present in between the coding sequence then they code for methionine and valine respectively. How does this happen? This is because of difference in initiator tRNA and the one used in between the process of translation (Figure 30.5). Initiator tRNA has unique features that distinguish from elongating tRNA in eubacteria. Firstly, there is no base pairing between 1st and 72nd bases in the acceptor arm. In anti-codon stem there are three consecutive GC base pairs. These base pairs are important to insert tRNA directly into P-site. These features are useful for interaction of this initiation tRNA with IF-2. Three GC base pairs are also found in Eukaryotes and archaebacteria just like in prokaryotes .Several initiation factors are required in the process, which include IF-1, IF-2, and IF-3.